Toll-like receptors (TLRs) are signaling receptors that are central to innate immune recognition of a variety of pathogens. Activation of TLR signaling by conserved microbial structures promotes the induction of innate immune responses. Such responses must be kept under tight control. Responses that are disrupted, delayed, or of insufficient vigor can lead to a failure to control infection. On the other hand, excessive or inappropriate inflammation can be harmful or even fatal- something seen in the hyper-activation of innate immune responses that marks microbial sepsis. Innate immune dysregulation is thought to be central to the pathogenesis of the diseases caused by diverse Category A and B biological threat agents. Conversely, well-regulated innate immune responses are thought to be key to successful host responses to these pathogens, as well as to successful vaccination against these and other infectious agents. Signaling through TLRs is tightly regulated. We recently discovered a novel mechanism of TLR counter- regulation- the TLR4 homolog, RP105. We have found that: (a) RP105 expression mirrors that of TLR4 on antigen presenting cells;(b) RP105 is a specific inhibitor of TLR4 signaling in HEK293 cells;(2) RP105 interacts directly with TLR4, inhibiting the ability of the TLR4 signaling complex to bind lipopolysaccharide (LPS);(3) RP105 regulates TLR4 signaling in dendritic cells and macrophages;(4) RP105 regulates in vivo responses to LPS;and (5) RP105 acts as a biologically important inhibitor of TLR4 signaling in both microbial and non-microbial models of inflammation. The central hypothesis underlying these studies is that RP105 is a physiological regulator of TLR4 signaling and TLR4-driven immune responses. The long-term goal of this work is to define the molecular mechanisms that underlie the generation and regulation of successful immune responses to biological threat agents and other pathogens. The studies in this proposal will characterize, both in vitro and in vivo, the capacity of RP105 to modulate TLR signaling and TLR-driven immune responses. We will: (1) Define the molecular mechanisms and biological consequences of RP105- mediated modulation of TLR signaling in myeloid cells;(2) Determine the mechanisms responsible for modulation of B cell function by RP105;and (3) Define the role of RP105 in regulating TLR4 signaling in response to endogenous ligands.